1. Field of the Invention
The invention relates to a method for reducing motion blur, flicker and loss of brightness of images shown in non-stroboscopic display devices, in which each image of an input video signal is displayed during a display time ti, which is less than or equal to a picture period T. The invention further relates to a non-stroboscopic display device.
2. Description of the Related Art
Non-stroboscopic non-emissive displays, such as Liquid Crystal Displays (LCD), Plasma Panel Displays (PDP), Thin Film Transistor (TFT) displays, Liquid Crystal on Silicon (LCOS) displays or Color Sequential Displays, consist of a display panel having a row and column array of picture elements (pixels) for modulating light, means for illuminating the display panel from the front or back side, and drive means for driving the pixels in accordance with an applied input video signal. Quite similar, non-stroboscopic emissive displays, such as Organic Light Emitting Diodes (O-LED) displays or Polymer Light Emitting Diodes (Poly-LED) displays, consist of a display panel having a row and column array of pixels (LEDs) and drive means for driving the pixel (LEDs) in accordance with an applied input video signal. However, the pixels (LEDs) emit and modulate light by themselves without requiring illumination from the front or back side.
In state-of-the-art Cathode Ray Tubes (CRTs), each pixel of a displayed image is generated as a pulse, which is very short compared to the picture period T. Different to these state-of-the-art CRTs, in new flat, high quality, low cost non-stroboscopic display devices, each pixel is displayed during most of the picture period. Of course, this non-stroboscopic behavior also holds for types of CRTs whose pixels, e.g., slow phosphor atoms, are active for a time not negligible to the picture period. In the sequel of this description, we thus will only differentiate between stroboscopic and non-stroboscopic displays, and in case of a non-stroboscopic display, we will use the term “pixel” for both the elements of a light modulation/generation array and the activated (slow) atoms of a CRT-type display.
In the case where any part of the image displayed on a non-stroboscopic display contains motion, the viewer will track this motion. As each pixel is displayed substantially the whole picture period, the intensity of pixels showing the motion is integrated along the motion trajectory as follows:
                                          F            out                    ⁡                      (                                          x                →                            ,              n                        )                          =                              1                          t              i                                ⁢                                    ∫              0                              t                i                                      ⁢                                          F                ⁡                                  (                                                                                    x                        →                                            +                                                                        t                          T                                                ⁢                                                  D                          →                                                                                      ,                    n                                    )                                            ⁢                                                          ⁢                              ⅆ                t                                                                        (        1        )            with ti as display time of each image, F as input video signal, FOUT as output video signal, and T as picture period. The motion vector D= vT is the product of the object velocity v and the picture period T. In case ti is constant, the integration is the same as a convolution of F( x,n) and a sample-and-hold function h(α):
                                                                                          F                  out                                ⁡                                  (                                                            x                      →                                        ,                    n                                    )                                            =                            ⁢                                                T                                      t                    i                                                  ⁢                                                      ∫                    0                                                                  t                        i                                            T                                                        ⁢                                                            F                      ⁡                                              (                                                                                                            x                              →                                                        +                                                          α                              ⁢                                                                                                                          ⁢                                                              D                                →                                                                                                              ,                          n                                                )                                                              ⁢                                                                                  ⁢                                          ⅆ                      α                                                                                                                                              =                            ⁢                                                ∫                                      -                    ∞                                    ∞                                ⁢                                                                            F                      ⁡                                              (                                                                                                            x                              →                                                        +                                                          α                              ⁢                                                                                                                          ⁢                                                              D                                →                                                                                                              ,                          n                                                )                                                              ·                                          h                      ⁡                                              (                        α                        )                                                                              ⁢                                                                          ⁢                                      ⅆ                    α                                                                                                          (        2        )                        where                                                                h          ⁡                      (            α            )                          =                  {                                                                                          T                                          t                      i                                                        ,                                                                              0                  ≤                  α                  ≤                                                            t                      i                                        T                                                                                                                        0                  ,                                                            otherwise                                                                        (        3        )            is a 1D block function, oriented along the motion vector D. It is therefore actually a 2D function h( x), which has zero value outside the line segment x=k D, 0≦k≦ti/T, while the 2D integral area is normalized to 1. The 2D spatial Fourier transform is:
                                                                                          F                  out                                ⁡                                  (                                                            f                      →                                        ,                    n                                    )                                            =                                                ∫                                      -                    ∞                                    ∞                                ⁢                                                      ∫                                          -                      ∞                                        ∞                                    ⁢                                                                                    F                        out                                            ⁡                                              (                                                                              x                            →                                                    ,                          n                                                )                                                              ⁢                                                                                  ⁢                                          ⅇ                                              (                                                                              -                            j                                                    ⁢                                                                                                          ⁢                          2                          ⁢                                                                                                          ⁢                          π                          ⁢                                                                                                          ⁢                                                      x                            →                                                    ⁢                                                      f                            →                                                                          )                                                              ⁢                                                                                  ⁢                                          ⅆ                                              x                        →                                                                                                                                                                    =                                                F                  ⁡                                      (                                                                  f                        →                                            ,                      n                                        )                                                  ·                                  H                  ⁡                                      (                                          f                      →                                        )                                                                                                          (        4        )            with F( f,n) the 2D spatial Fourier transform of the original signal F( x,n), and H( f) the 2D spatial Fourier transform of H( x):
                              H          ⁡                      (                          f              →                        )                          =                                            sin              ⁡                              (                                  π                  ⁢                                                                          ⁢                                      D                    →                                    ⁢                                                                          ⁢                                                            t                      i                                        T                                    ⁢                                      f                    →                                                  )                                                    π              ⁢                                                          ⁢                              D                →                            ⁢                                                          ⁢                                                t                  i                                T                            ⁢                              f                →                                              .                                    (        5        )            
Apparently, the effect of the motion tracking/temporal sample-and-hold characteristic is a low-pass filtering in the direction of the motion with a sinc-frequency response, with a cut-off-frequency being inversely proportional to the quantity
                    t        i            T        ⁢          D      →        ,where
      t    i    Tis denoted as the duty cycle of the display. The non-stroboscopic light generation, combined with the eye tracking of the viewer trying to follow moving objects from one image to the next, thus leads to the perception of motion-dependent blur in the images. When the motion D in the image increases, the cut-off-frequency of the spatial low-pass filter and thus the degree of perceived motion blur can be kept constant by reducing the display time ti (or the duty cycle
      t    i    Twith the drawback of loss of brightness and increased flicker.
U.S. Patent Application Publication No. U.S. 2002/0003522 A1, corresponding to U.S. Pat. No. 7,106,350 B2, discloses that motion blur in non-stroboscopic display devices can be mitigated by reducing the display time ti of each image. The display time in light modulating displays can be efficiently controlled by switching the lamps that illuminate the display panel, or by appropriate driving of a shutter element that is able to block the light flux through the display panel. In emissive O-LED or Poly-LED displays, the display time is even simpler controlled by switching the LEDs themselves. The reduction of the display time increases the cut-off-frequency of the low-pass filter in the spatial frequency domain, so that less image information in the spatial frequency domain is suppressed and less motion blur occurs. U.S. 2002/0003522 A1 proposes to decide whether an image is a motion image or a still image. The display time ti is then assigned one of two predefined values according to the threshold-based outcome of the binary decision, e.g., ti=T/2 for the motion image and ti=T for the still image.
The general disadvantage encountered when reducing the display time to reduce motion blur is the accompanying reduction of image brightness. Furthermore, the presence of display (ti) and non-display (T-ti) periods within a picture period of duration T are perceived as flicker by the viewer. Reduced motion blur is thus traded against increased flicker and loss of brightness.
The adjustment of the display time based on a binary decision whether an image of a motion image or a still image as proposed in U.S. 2002/0003522 A1 leads to the adjustment of the same display time for images with a large amount of motion and for images with a medium amount of motion that is only large enough so that the image is considered as motion image, depending on the threshold defined for the decision between motion images and still images. Thus, especially for said image with medium amount of motion near the decision threshold, by far too much flicker and loss of brightness is accepted than actually necessary. In a similar fashion, images with medium amount of motion near the decision threshold that are considered as still images undergo by far too much blur than actually necessary.